1. Field of the Invention
The United States produces more than 6 billion lb. of plastic film each year, virtually all of which is made from petroleum-based raw materials. These films are made most economically by the extrusion blowing process in which a tubular extruded bubble is expanded and shaped by air streams at the exit of the extruder die. In many packaging applications, it is desirable that a film be shrinkable so that it can be made to fit the contours of irregularly shaped articles. Shrink films are now widely used in the packaging industry.
The long-term uncertainty of prices and supplies of petroleum has sustained interest in alternative sources of raw materials for making plastics. This invention relates to the preparation of shrink films by the blowing of formulations based on a renewable resource.
2. Description of the Prior Art
In U.S. Pat. No. 4,026,849, Bagley et al. teach the preparation of plastic articles by extrusion of compositions comprising graft copolymers of starch and thermoplastic polymers such as poly(methyl acrylate). The resultant products are water-resistant, yet biodegradable; and they conserve valuable petroleum resources, because the starch portion serves as an extender for the petroleum-based polymer. Dennenberg et al. [J. Appl. Polym. Sci. 22: 459-465 (1978)]teach that starch graft poly(methyl acrylate) copolymers can be extruded into products having excellent initial tensile strength and elongation properties. However, simple extrusion is a relatively slow process that is considerably more expensive than the more conventional extrusion blowing technique. We found that this graft copolymer of unmodified starch and methyl acrylate can not be extrusion blown to give continuous films because of its particulate nature.
The shrink films now available commercially require the application of heat to the film. Heat shrinkability arises from an elastic memory imparted to some thermoplastic films during their manufacture by either stretch orientation or by cross-linking induced through irradiation. Shrinkage takes place when heat is applied to the film, and it tends to revert to its original unoriented state. Films that can be shrunk by simple exposure to high relative humidity at room temperature have not been reported.